The present invention relates to X-ray imaging apparatus, and more specifically to means for depressing transient current surges through an X-ray tube of the apparatus and for reducing radio frequency emissions produced by such current surges.
The X-ray imaging apparatus includes a vacuum tube with a cathode and anode that emits X-rays during operation. The cathode comprises tungsten thermionic emitting source and focusing surfaces. The cathode is part of an assembly which includes a filament to heat the cathode to an operating temperature. Upon application of a potential across the electrodes of the X-ray tube, thermionically emitted electrons traverse the vacuum gap between the cathode and anode, impacting the anode thereby generating X-rays.
A major problem during the operation of X-ray tubes is high voltage discharge or arcing between the electrodes due to intense electric field gradients caused by contamination or rough edges on the surfaces of the electrodes. These discharges, commonly known as "spits", cause radiated and conducted electrical noise of great intensity which can interfere with the operation of electronic circuitry in the vicinity of the tube. In extreme cases, electrical noise from the spits even causes failure of semiconductor devices in adjacent equipment.
A newly manufactured tube is subject to frequent and prolonged spitting which must be greatly reduced in order to be a usable product. Each time a spit occurs, some material around the point that caused the intense field gradient is vaporized. As part of the manufacturing process, a new X-ray tube is "seasoned" by allowing spits to smooth the electrodes by vaporizing any foreign particles and surface roughness that can cause intense field gradients.
The seasoning process is affected by the energy available to vaporize the material and by the way the energy is delivered to the discharge arc. If too much energy is delivered, the imperfection is vaporized along with underlying material, sometimes forming a crater whose rim may have edges sharp enough to cause additional spits and more extensive erosion of the electrode. In conventional seasoning the energy available to the spit is determined by the voltage and capacitance of the high voltage cables feeding the tube and is typically in the range of tens of joules. The current is determined by the voltage and characteristic impedance of the cables and can be one thousand amperes or more.
A limiting resistor has been connected in series with the anode of the tube to try to control the peak current of the discharge. A problem with this technique is that the stored energy in the high voltage cables is discharged into both the arc and the resistor in an uncontrolled ratio. The resistor and the arc are in series and thus have the same current. The arc has a hyperbolic negative resistance volt-ampere characteristic and the resistor has a linear positive resistance characteristic which results in the two sharing the source voltage and power in an unstable, oscillatory manner. The energy that actually is delivered to the vaporization process is somewhat random and difficult to control with a resistor.
Even when an X-ray tube is properly seasoned during manufacture, these discharges occasionally occur while the tube is operating in an imaging apparatus. The discharges shorten the life of the tube, as well as producing electrical noise. The discharges become more and more frequent as the tube nears the end of its useful life and is one of its major failure modes.